1. Field of the Invention
This invention relates to a liquid crystal display device using a horizontal electric field, and more particularly to a thin film transistor substrate of horizontal electric field applying type and a fabricating method thereof that are adaptive for simplifying a process.
2. Discussion of the Related Art
Generally, a liquid crystal display (LCD) controls light transmittance of a liquid crystal having a dielectric anisotropy using an electric field to thereby display a picture. To this end, the LCD includes a liquid crystal display panel for displaying a picture by a liquid crystal cell matrix, and a driving circuit for driving the liquid crystal display panel.
In FIG. 1, a related art liquid crystal display panel is comprised of a color filter substrate 10 and a thin film transistor substrate 20 that are joined to each other with a liquid crystal 24 therebetween.
The color filter substrate 10 includes a black matrix 4, a color filter 6 and a common electrode 8 that are sequentially provided on an upper glass substrate 2. The black matrix 4 is provided in a matrix type on the upper glass substrate 2. The black matrix 4 divides an area of the upper glass substrate 2 into a plurality of cell areas to be provided with the color filter 6, and prevents a light interference between adjacent cells and an external light reflection. The color filter 6 is provided at the cell area divided by the black matrix 4 in such a manner to be divided into red (R), green (G) and blue (B) areas, thereby transmitting red, green and blue lights. The common electrode 8 is formed of a transparent conductive layer coated entirely on the color filter 6, and supplies a common voltage Vcom that serves as a reference voltage upon driving of the liquid crystal 24. Further, an overcoat layer (not shown) for smoothing the color filter 6 may be provided between the color filter 6 and the common electrode 8.
The thin film transistor substrate 20 includes a thin film transistor 18 and a pixel electrode 22 provided for each cell area defined by a crossing between a gate line 14 and a data line 16 at a lower glass substrate 12. The thin film transistor 18 applies a data signal from the data line 16 to the pixel electrode 22 in response to a gate signal from the gate line 14. The pixel electrode 22 formed from a transparent conductive layer supplies a data signal from the thin film transistor 18 to drive the liquid crystal 24.
The liquid crystal 24 having a dielectric anisotropy is rotated in accordance with an electric field formed by a data signal from a pixel electrode 22 and a common voltage Vcom from the common electrode 8 to control light transmittance, thereby implementing a gray scale level.
Further, a liquid crystal display panel includes a spacer (not shown) for constantly keeping a cell gap between the color filter substrate 10 and the thin film transistor substrate 20.
In the liquid crystal display panel, the color filter substrate 10 and the thin film transistor substrate 20 are formed by a plurality of mask processes. Herein, one mask process includes many processes such as thin film deposition (coating), cleaning, photolithography, etching, photo-resist stripping and inspection processes, etc.
In particular, because the thin film transistor substrate includes the semiconductor process and requires the plurality of mask processes, it has a complicated fabricating process which is a major factor in increased manufacturing costs of the liquid crystal display panel. Therefore, the thin film transistor substrate has been developed toward a reduction in the number of mask process from a five-round mask process that is a standard mask process.
Meanwhile, the liquid crystal displays are largely classified into a vertical electric field applying type and a horizontal electric field applying type based upon a direction of the electric field driving the liquid crystal.
The liquid crystal display device of a vertical electric field applying type drives a liquid crystal in a twisted nematic (TN) mode with a vertical electric field formed between a pixel electrode and a common electrode arranged opposite to each other on the upper and lower substrates. The liquid crystal display device of a vertical electric field applying type has an advantage of a large aperture ratio; while having a drawback of a narrow viewing angle of about 90°.
The liquid crystal display device of a horizontal electric field applying type drives a liquid crystal in an in-plane switching (IPS) mode with a horizontal electric field between the pixel electrode and the common electrode arranged in parallel to each other on the lower substrate. The liquid crystal display device of a horizontal electric field applying type has an advantage of a wide viewing angle of about 160°.
The thin film transistor substrate in the liquid crystal display device of horizontal electric field applying type also requires a plurality of mask processes which leads to a drawback of a complicated fabricating process. Therefore, in order to reduce the manufacturing cost, it is necessary to reduce the number of mask processes.